1. Field of the Invention
The present invention relates to a Faraday rotator formed of a bismuth-substituted rare earth-iron-garnet single crystal and capable of reversibly controlling the plane of polarization through it, and relates to an optical device that comprises it (e.g., optical isolator, optical circulator, optical switch, optical attenuator, polarization controller).
The invention also relates to an antireflection film formed on the surface of a Bi-substituted rare earth-iron-garnet material to be used for such a Faraday rotator, and to an optical device that comprises it.
2. Description of the Related Art
First Related Art:
A Bi-substituted rare earth-iron-garnet film that has grown in a mode of liquid-phase epitaxial (LPE) growth has heretofore been much used for Faraday rotators for optical isolators in optical communication systems. Apart from their application to such optical isolators, in addition, Faraday rotators are being utilized these days also for magneto-optic-type optical attenuators, optical switches and polarization controllers in WDM (wavelength division multiplexing) systems (for example, see JP-A 6-51255).
In an optical isolator, an external magnetic field is applied to the Faraday rotator or the Faraday rotator itself is a permanent magnet. In driving such an optical isolator, the light traveling direction and the magnetic moment direction therein are controlled to be almost the same to thereby produce a predetermined Faraday rotation angle.
On the other hand, in other optical devices having a Faraday rotator such as magneto-optic-type attenuators, an external magnetic field H that is opposite to the light-traveling direction therein is applied to the Faraday rotator in order that the resulting Faraday rotation angle may be reversibly changed.
In that case, the intensity of the external magnetic field H to be applied to the Faraday rotator is controlled to be comparable to or larger than that of the saturation magnetic field Hs of the Faraday rotator to thereby reduce the light diffraction loss to be caused by the magnetic domain structure of the Faraday rotator. When the crystal for a Faraday rotator is grown in a mode of liquid-phase epitaxial growth, it receives magnetic anisotropy in the growing direction (growth-induced magnetic anisotropy), and this produces an axis of easy magnetization (easy axis) in the crystal-growing direction in which, therefore, the Faraday rotator formed of the crystal is easily magnetized. Accordingly, regarding a Faraday rotator of a type of which the light-incident surface is almost perpendicular to the crystal-growing direction and which is not subjected to heat treatment, even when an external magnetic field H is applied to it in the direction that is oblique to the light-input surface thereof, the magnetic moment orientation of the resulting Faraday rotator does not almost shift from the crystal-growing direction thereof.
Given that situation, a Faraday rotator is heated at a high temperature of around 1000° C. so as to reduce the growth-induced magnetic anisotropy thereof. Then, its easy axis is not in the direction of <111> that is the same as the crystal-growing direction but is in a different direction of <111> that is nearer to the plane direction of the growth surface owing to the effect of the shape magnetic anisotropy of the crystal that forms the Faraday rotator. As in FIG. 24, there are four <111> directions on a substrate. The first direction is a <111> direction a that is perpendicular to the substrate surface; and the remaining three directions <111> β1, β2 and β3 each have an angle of about 20° from the substrate surface. From the <111> direction α, the angle between β1 and β2, that between β2 and β3 and that between β3 and β1 are all 120°. When the growth-induced magnetic anisotropy reduces, then the magnetic moment direction is readily oriented toward the direction that is parallel to the substrate surface due to the formed effect of an epitaxial film growing in a disk style. Therefore, the magnetic moment orientation shall be in the three directions β1, β2 and β3 that are the nearest to the direction parallel to the substrate surface. Accordingly, it is possible to change the magnetic moment direction of the Faraday rotator in accordance with the direction in which the external magnetic field H that is higher than the saturation magnetic field Hs is applied to the Faraday rotator, and the rotation angle of the Faraday rotator is thereby changeable.
FIG. 8 is a graph showing the relationship between the external magnetic field H applied to a Faraday rotator and the Faraday rotation angle. In this, the horizontal axis indicates the external magnetic field H (Oe); and the vertical axis indicates the Faraday rotation angle (deg.). The curve a shows the Faraday rotation angle of a heat-treated Faraday rotator in an external magnetic field H; and the curve β shows the Faraday rotation angle of a non-heated Faraday rotator in the same external magnetic field H. The value Hsβ on the horizontal axis indicates the intensity of the saturation magnetic field Hs of the non-heated Faraday rotator; and the value Hsα thereon indicates the intensity of the saturation magnetic field Hs of the heat-treated Faraday rotator. In the case of FIG. 8, the external magnetic field H is applied to the Faraday rotator in the direction in which the garnet single crystal to form the Faraday rotator has grown, and this is in the direction in which light enters or goes out of the Faraday rotator.
With the growth-induced magnetic anisotropy decreasing, the magnetic moment orientation may shift more easily in any desired direction in accordance with the change of the direction of the external magnetic field H. Heated at a higher temperature for a longer period of time, crystals more readily undergo atomic rearrangement therein, and their growth-induced magnetic anisotropy reduces more. On the other hand, however, their saturation magnetic field Hs increases more. As a result, therefore, the saturation magnetic field Hsα of the Faraday rotator, which was heated to reduce the growth-induced magnetic anisotropy thereof, is significantly larger than the saturation magnetic field Hsβ of the non-heated Faraday rotator, as in FIG. 8.
For the reasons as above, the magnet (permanent magnet or electromagnet) that generates the external magnetic field H enough to saturate the Faraday rotator shall be large-sized. In addition, the electromagnet that generates the variable magnetic field to constitute the synthetic magnetic field, which is for changing the Faraday rotation angle, shall be also large-sized, and, as a result, a large current must be flowed to the coil of the electromagnet. This brings about some problems in that the optical devices that comprise the Faraday rotator and the magnetic circuit for it are inevitably large-sized and their production costs increase. Another problem with the Faraday rotator of the type is that, if the Faraday rotator is poorly heat-treated, it still has the growth-induced magnetic anisotropy remaining therein and, if so, its magnetic moment orientation does not move even when the direction of the external magnetic field H applied thereto is changed, and therefore the Faraday rotation angle does not change satisfactorily.
In this description, the uppermost limit of the magnetic field that does no more increase the Faraday rotation angle of a Faraday rotator even though the intensity of the magnetic field applied to the Faraday rotator is further increased is referred to as the saturation magnetic field Hs of the Faraday rotator.
Second Related Art:
As so mentioned hereinabove, the Bi-substituted rare earth-iron-garnet single-crystal film that has grown in a mode of liquid-phase epitaxial growth shall undergo growth-induced magnetic an isotropy in the film-growing direction. Accordingly, the magnetic moment orientation of the garnet single-crystal film is fixed to be the same as the epitaxial growth direction. In general, the Faraday rotator to be used in optical isolators is so designed that its magnetic moment orientation shall be the same as the epitaxial growth direction of the garnet single-crystal film that forms it. Accordingly, for the Faraday rotator for that use, the characteristic magnetic property of the epitaxially-grown garnet single-crystal film causes no problem in practical use.
Contrary to this, however, for the Faraday rotator to be in variable optical attenuators in which the Faraday rotation angle shall be variable, the garnet single-crystal film must be so processed that its magnetic moment orientation is inclined relative to the epitaxial growth direction of the garnet single-crystal film. For this, a magnetic field is applied to the garnet single-crystal film in the direction that differs from the epitaxial growth direction of the garnet single-crystal film. In this case, strong growth-induced magnetic anisotropy, if any, of the garnet single-crystal film will be a bar to the intended change of the magnetic moment orientation of the film in the inclined direction. Accordingly, the Bi-substituted rare earth-iron-garnet single-crystal film is heated at a high temperature not lower than 1000° C. to thereby weaken the growth-induced magnetic anisotropy thereof in order that the magnetic moment direction of the thus-heated film could be oriented to the direction of the magnetic field applied to the film and therefore the Faraday rotation angle of the Faraday rotator formed of the film could be variable (for example, see JP-A 10-1398).
When the growth-induced magnetic anisotropy of the heat-treated garnet single-crystal film is weakened, the magnetic moment direction of the film is readily oriented in the direction except the film growth direction, or that is, it is hardly oriented in the film growth direction. Accordingly, the saturation magnetic field Hs in the film growth direction of the garnet single-crystal film is larger after heat treatment of the film than before heat treatment thereof. With the intensity of the magnetic field applied to the film in the film growth direction being gradually increased from 0, the Faraday rotation angle of the Faraday rotator formed of the film is measured, and the magnetic field in which the Faraday rotation angle no more changes is referred to as the saturation magnetic field Hs. FIG. 2 shows the condition of a Faraday rotator 1 that has received an external magnetic field H (not shown) lower than the saturation magnetic field Hs thereof in the direction almost perpendicular to the light-input surface of the Faraday rotator 1. As in FIG. 2, a part of the magnetic moment 2 of the Faraday rotator 1 is directed toward the direction of the magnetic field applied to the Faraday rotator 1, while the other thereof is opposite to it. Therefore, in this condition, the garnet single-crystal film that forms the Faraday rotator 1 shall have a different magnetic domain structure.
When a specifically polarized light Ii is incident to the Faraday rotator 1 that is in an external magnetic field H lower than the saturation magnetic field Hs thereof, then it is further polarized differently in the region of the Faraday rotator in which the direction of the magnetic moment 2 thereof is opposite to each other. As a result, the light having entered the Faraday rotator 1 is diffracted to scatter therein to give diffracted light Ir, and therefore the output light Io is thereby reduced, as shown in FIG. 2. This is light loss through the Faraday rotator 1, and this increases the light loss in optical devices that comprise the Faraday rotator 1.
Accordingly, the condition for the heat treatment of the garnet single-crystal film for weakening the growth-induced magnetic anisotropy thereof must be so controlled that the direction of the magnetic moment 2 of the heated film may become readily changed when the film has received an external magnetic field H applied thereto and, in addition, the saturation magnetic field Hs of the film does not increase as much as possible. However, the Faraday rotator that has been heat-treated in the thus-controlled condition to satisfy the requirement will come up against a problem in that the Faraday rotation angle could not accurately vary within a desired range. This is because of the reduction in the magnetic moment orientation reproducibility of the Faraday rotator to which is applied a magnetic field for orienting the magnetic moment direction thereof toward the direction of the magnetic field applied thereto. This problem after all leads to another problem in that magneto-optic-type optical attenuators that comprise the Faraday rotator of the type could not enjoy satisfactory attenuation.
Third Related Art:
An antireflection film is an optical thin film formed on the surface of an optical device via which light goes in or goes out of the structure, and it acts to prevent light reflection on the interface between the optical device and a substance of which the refractive index differs from that of the optical device.
In various optical devices used in optical communication systems, such an antireflection film is formed in the interface through which light runs therein, and it acts to prevent light from reflecting thereon to thereby reduce the returning light. Also in the Faraday rotator that is used for optical isolators or optical attenuators which are passive devices in optical communication systems, an antireflection film is formed on both surfaces thereof through which light goes in or goes out of it. With the antireflection film of the type, the Faraday rotator is built in the intended devices. In the Faraday rotator, concretely, such an antireflection film is formed in the interface between the magnetic garnet, or that is, the constitutive material of the Faraday rotator and air, or in the interface between the magnetic garnet and an epoxy resin. In the latter, the epoxy resin is used for bonding the Faraday rotator to any other optical device, and light runs through the bonding interface of the two.
In general, such an antireflection film is composed of thin films of materials of different refractivity that are formed through vapor deposition, for example, as in JP-A4-230701. Some conventional antireflection films are specifically so designed that they have a lowered refractive index selectively to light of a specific wavelength, for example, a wavelength λ of 1310 nm or 1550 nm in optical communication systems.
With the drastic increase in communication data of these days, wavelength division multiplexing (WDM) systems are now employed in the recent optical communication technology, in which multiple wavelengths λ of light are used for the purpose of remarkably increasing the communication capacity in the optical communication systems. In the wavelength division multiplexing systems light of different wavelengths diffuses in a wide area and passes through optical fibers and passive devices, as compared with light of a single wavelength in ordinary optical communication systems. According to the current optical communication technology, however, the Faraday rotator having, for example, a Faraday rotation angle of 45 degrees relative to the light having a wavelength of 1550 nm is worked to have an antireflection film of low reflectivity for the single wavelength light of 1550 nm. Having the antireflection film of the type, therefore, the Faraday rotator is unsatisfactory in point of the antireflection to any other light than the light of 1550 nm. Therefore, this causes a problem in that the light except 1550 nm is reflected on the light-input surface of the Faraday rotator to form a returning light and the insertion loss of the Faraday rotator therefore increases.
Multiple wavelengths are used in the wavelength division multiplex system, and the Faraday rotators to be used for their light sources are so designed that their rotation angle to the wavelength specifically defined for them should be 45 degrees. In this connection, the characteristic property of magnetic garnet for such Faraday rotators must be taken into consideration. Specifically, the Faraday rotation angle of magnetic garnet varies depending on the wavelength of light that enters it. Therefore, different Faraday rotators each satisfying the rotation angle of 45 degrees for the intended different wavelengths must be fabricated for the wavelength division multiplex system. For this, in conventional vapor deposition of forming antireflection films, the optimum antireflection films for the intended different wavelengths are formed for all Faraday rotators that are fabricated for different wavelengths. The antireflection films are formed in a batch process of vapor deposition such as vacuum evaporation or the like, and the increase in the number of the wavelengths for the antireflection films to be formed through such vapor deposition inevitably results in the increase in the number of the film-forming operations. This is problematic in that the process of forming the antireflection films is complicated and its productivity lowers.